Transmissible spongiform encephalopathies (TSE) (prion disease, scrapie) are fatal neurodegenerative diseases that occur in humans and animals. This grant's focus is on elucidating the pathogenic mechanism(s) contributing to the neuronal injury and on utilizing molecular approaches to treat and/or about disease. We hypothesize that the disease of the CNS characterize by spongiosis, astrocytosis, neural death, neural dropout and accumulation of an abnormal isoform (PrP/sc) from the host-encoded cellular PrP (PrP/c) occurs by two distinct pathways. The first is by direct accumulation of PrP/sc in neurons. However, the presence of exogenous PrP/c delays/aborts the onset of disease. A major aim of this proposal is to continue studies and to expand the evaluation of exogenous PrP/c in aborting/treating TSE disease. The second major aim of this proposal evaluates the possibility that neuronal injury also occurs from bystander (indirect) effects. TNF- alpha, a cytokine-implicated in apoptosis, is markedly increased in the CNS during scrapie disease. TNF-alpha expression parallels the onset of clinical symptoms and experiments using TNF-alpha ko mice are designed to determine its role in disease. In addition, in prion + (mPrP+) but not in prion/null (ko) (mPrP/null) mice, inoculation of murine scrapie leads to infiltration of T lymphocytes into the brain that parallels the clinical course of disease. Initial experiments show it takes a significantly longer time for MHC class I ko mice given murine scrapie agent to develop clinical and histological scrapie when compared to MHC class I sufficient mice, strongly suggesting that CD8+ T cells may contribute to the pathogenesis of scrapie when compared to MHC class I sufficient mice, strongly suggesting that CD8+ T cells may contribute to the pathogenesis of scrapie. The enigma of immune tolerance to PrP is to be re-evaluated. The role played by MHC molecules and T cell subsets in the pathogenesis of TSE disease will be investigated, and regions on PrP recognized by T cell determined.